The present invention relates to the field of fuel cell power plants and, more particularly, to a method and apparatus for maintaining water balance of the power plant in a wider envelope of operating conditions.
Fuel cell power plants are well known and are commonly used to produce electrical energy from reducing and oxidizing fluids to provide electricity. In such power plants, a plurality of planar fuel cells are typically arranged in a stack surrounded by an electrically insulating frame that defines manifolds for directing flow of the reducing and oxidizing fluids, as well as coolant and product or exhaust fluids. Each individual cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. A reactant or reducing fluid such as hydrogen or other hydrogen rich fuel is supplied to the anode electrode, and an oxidant such as oxygen or air, typically ambient air, is supplied to the cathode electrode.
In proton exchange membrane (PEM) fuel cells, the electrolyte is saturated with water in order to provide high ion conductivity across the membrane. Other components such as the anode electrode adjacent to the electrolyte must remain partially saturated to prevent membrane dry out from the movement of water in the cell. As the power plant is operated, however, water is formed and/or transported to the cathode and removed from the cathode, typically in the exhaust stream exiting the cathode and/or through a water transport plate as shown in commonly owned U.S. Pat. No. 5,853,909.
An operational limit on performance of a fuel cell is defined by an ability of the cell to maintain water balance as electrical current drawn from the cell into the external load circuit varies and as an operating environment of the cell varies. For proton exchange membrane (PEM) electrolyte fuel cells, if sufficient water is not returned to the anode electrode, adjacent portions of the PEM electrolyte dry out thereby decreasing the rate in which hydrogen ions may be transferred through the PEM and also resulting in crossover of the reducing fluid leading to local overheating. Similarly, if insufficient water is removed from the cathode, the cathode may become flooded effectively limiting oxidant supplied to the cathode and hence decreasing current flow. Further, if too much water is removed from the cathode in the exhaust stream, the membrane adjacent to the cathode may dry out limiting ability of hydrogen ions to pass through the PEM, thus decreasing cell performance.
When the fuel cell power plant is to be used in transportation vehicles, the problem of maintaining an efficient water balance is exacerbated as non-plant sources of water are no longer available during use, and thus, sufficient water must be retained within the plant to offset losses from reactant fluids exiting the plant. In order to operate in an acceptable manner, therefore, the plant must be operated such that water produced electrochemically at the cathode and water retained within the plant balance against water removed from the exhaust stream.
Although numerous efforts have been made to provide effective water balance, particularly in operating conditions where ambient air is used as the oxidant and is frequently available only at high temperatures, the need remains for an effective and efficient mechanism for keeping the fuel cell power plant in water balance.
It is therefore the primary object of the present invention to provide a fuel cell power plant and operating method which reliably achieve water balance at widely varying operating conditions.
Other objects and advantages of the present invention will appear hereinbelow.
In accordance with the present invention, the foregoing objects and advantages have been readily attained. According to the invention, a fuel cell power plant with enhanced water recovery is provided, which comprises a fuel cell power plant adapted to receive a reducing fluid and an oxidant and to generate therefrom electricity and an at least partially saturated exhaust stream; a mass and energy transfer device defining a first flow passage for said exhaust stream and a second flow passage for an oxidant stream, said first flow passage being in mass transfer relationship with said second flow passage; and means for cooling at least one of said oxidant stream, said exhaust stream and said mass and energy transfer device, whereby water is transferred from the exhaust stream to the oxidant stream so as to produce an at least partially saturated oxidant stream.
In further accordance with the invention, the fuel cell power plant components may be operated at pressures above ambient to further reduce the power plant""s water loss in the exhaust stream. Further still the power plant can be operated such that a compressor is used to pressurize the power plant exhaust after it has passed through the mass and energy transfer device in order to enhance the water removal from the exhaust stream.
In further accordance with the invention, a method is provided for operating a fuel cell power plant, and specifically for recovering water from the exhaust stream from a fuel cell power plant, which method comprises the steps of (i) feeding a reducing fluid and an oxidant to a fuel cell power plant so as to generate electricity and an at least partially saturated exhaust stream; (ii) providing a mass and energy transfer device defining a first flow passage and a second flow passage and having a gas transfer barrier disposed therebetween; (iii) feeding said at least partially saturated exhaust stream to said first flow passage; (iv) feeding an oxidant stream to said second flow passage; and (v) cooling at least one of said oxidant stream, said exhaust stream and said mass and energy transfer device, whereby water is transferred from said at least partially saturated exhaust stream to said oxidant stream so as to produce an at least partially saturated oxidant stream.